Email Address Internationalization Y. Abel
(EAI) TWNIC
Internet-Draft S. Steele
Obsoletes: 5335 (if approved) Microsoft
Updates: 2045, 5322 July 05, 2010
(if approved)
Intended status: Standards Track
Expires: January 6, 2011
Internationalized Email Headers
draft-ietf-eai-rfc5335bis-00
Abstract
Full internationalization of electronic mail requires not only the
capabilities to transmit non-ASCII content, to encode selected
information in specific header fields, and to use non-ASCII
characters in envelope addresses. It also requires being able to
express those addresses and the information based on them in mail
header fields. This document specifies an variant of Internet mail
that permits the use of Unicode encoded in UTF-8, rather than ASCII,
as the base form for Internet email header field. This form is
permitted in transmission only if authorized by an SMTP extension, as
specified in an associated specification. This specification Updates
section 6.4 of [RFC2045] to conform with the requirements.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 6, 2011.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Role of This Specification . . . . . . . . . . . . . . . . 3
1.2. Relation to Other Standards . . . . . . . . . . . . . . . 3
2. Background and History . . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Changes on Message Header Fields . . . . . . . . . . . . . . . 5
4.1. UTF-8 Syntax and Normalization . . . . . . . . . . . . . . 5
4.2. Changes on MIME Headers . . . . . . . . . . . . . . . . . 6
4.3. Syntax Extensions to RFC 5322 . . . . . . . . . . . . . . 6
4.4. Change on addr-spec Syntax . . . . . . . . . . . . . . . . 8
4.5. Trace Field Syntax . . . . . . . . . . . . . . . . . . . . 8
4.6. message/global . . . . . . . . . . . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 10
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
8. Edit history . . . . . . . . . . . . . . . . . . . . . . . . . 12
8.1. draft-ietf-eai-rfc5335bis-00 . . . . . . . . . . . . . . . 12
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
9.1. Normative References . . . . . . . . . . . . . . . . . . . 12
9.2. Informative References . . . . . . . . . . . . . . . . . . 13
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1. Introduction
1.1. Role of This Specification
Full internationalization of electronic mail requires several
capabilities:
o The capability to transmit non-ASCII content, provided for as part
of the basic MIME specification [RFC2045], [RFC2046].
o The capability to use international characters in envelope
addresses, discussed in [I-D.ietf-eai-frmwrk-4952bis] and
specified in [I-D.yao-eai-rfc5336bis].
o The capability to express those addresses, and information related
to them and based on them, in mail header fields, defined in this
document.
This document specifies an variant of Internet mail that permits the
use of Unicode encoded in UTF-8 [RFC3629], rather than ASCII, as the
base form for Internet email header fields. This form is permitted
in transmission, if authorized by the SMTP extension specified in
[I-D.yao-eai-rfc5336bis] or by other transport mechanisms capable of
processing it.
1.2. Relation to Other Standards
This document updates Section 6.4 of [RFC2045]. It removes the
blanket ban on applying a content-transfer-encoding to all subtypes
of message/, and instead specifies that a composite subtype MAY
specify whether or not a content-transfer-encoding can be used for
that subtype, with "cannot be used" as the default.
This document also updates [RFC5322] and MIME ([RFC2045]), and people
who participate in the experiment have to swich to this document.
Allowing use of a content-transfer-encoding on subtypes of messages
is not limited to transmissions that are authorized by the SMTP
extension specified in [I-D.yao-eai-rfc5336bis]. Message/global (see
Section 4.6) permits use of a content-transfer-encoding.
2. Background and History
Mailbox names often represent the names of human users. Many of
these users throughout the world have names that are not normally
expressed with just the ASCII repertoire of characters, and would
like to use more or less their real names in their mailbox names.
These users are also likely to use non-ASCII text in their common
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names and subjects of email messages, both received and sent. This
protocol specifies UTF-8 as the encoding to represent email header
field bodies.
The traditional format of email messages [RFC5322] allows only ASCII
characters in the header fields of messages. This prevents users
from having email addresses that contain non-ASCII characters. It
further forces non-ASCII text in common names, comments, and in free
text (such as in the Subject: field) to be encoded (as required by
MIME format [RFC2047]). This specification describes a change to the
email message format that is related to the SMTP message transport
change described in the associated document
[I-D.ietf-eai-frmwrk-4952bis] and [I-D.yao-eai-rfc5336bis], and that
allows non-ASCII characters in most email header fields. These
changes affect SMTP clients, SMTP servers, mail user agents (MUAs),
list expanders, gateways to other media, and all other processes that
parse or handle email messages.
As specified in [I-D.yao-eai-rfc5336bis], an SMTP protocol extension
"UTF8SMTPbis" is used to prevent the transmission of messages with
UTF-8 header fields to systems that cannot handle such messages.
[[Note in Draft: Keyword related to UTF8SMTP will be decided by WG
before publication.]]
Use of this SMTP extension helps prevent the introduction of such
messages into message stores that might misinterpret, improperly
display, or mangle such messages. It should be noted that using an
ESMTP extension does not prevent transferring email messages with
UTF-8 header fields to other systems that use the email format for
messages and that may not be upgraded, such as unextended POP and
IMAP servers. Changes to these protocols to handle UTF-8 header
fields are addressed in [RFC5721]-bis and [RFC5738]-bis.
The objective for this protocol is to allow UTF-8 in email header
fields.
3. Terminology
A plain ASCII string is also a valid UTF-8 string; see [RFC3629]. In
this document, ordinary ASCII characters are UTF-8 characters if they
are in headers which contain <utf8-xtra-char>s.
Unless otherwise noted, all terms used here are defined in [RFC5321],
[RFC5322], [I-D.ietf-eai-frmwrk-4952bis],or [I-D.yao-eai-rfc5336bis].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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4. Changes on Message Header Fields
SMTP clients can send header fields in UTF-8 format, if the
UTF8SMTPbis extension is advertised by the SMTP server or is
permitted by other transport mechanisms.
This protocol does NOT change the [RFC5322] rules for defining header
field names. The bodies of header fields are allowed to contain
UTF-8 characters, but the header field names themselves must contain
only ASCII characters.
To permit UTF-8 characters in field values, the header definition in
[RFC5322] must be extended to support the new format. The following
ABNF is defined to substitute those definitions in [RFC5322].
The syntax rules not covered in this section remain as defined in
[RFC5322].
4.1. UTF-8 Syntax and Normalization
UTF-8 characters can be defined in terms of octets using the
following ABNF [RFC5234], taken from [RFC3629]:
UTF8-xtra-char = UTF8-2 / UTF8-3 / UTF8-4
UTF8-2 = %xC2-DF UTF8-tail
UTF8-3 = %xE0 %xA0-BF UTF8-tail /
%xE1-EC 2(UTF8-tail) /
%xED %x80-9F UTF8-tail /
%xEE-EF 2(UTF8-tail)
UTF8-4 = %xF0 %x90-BF 2( UTF8-tail ) /
%xF1-F3 3( UTF8-tail ) /
%xF4 %x80-8F 2( UTF8-tail )
UTF8-tail = %x80-BF
These are normatively defined in [RFC3629], but kept in this document
for reasons of convenience.
See [RFC5198] for a discussion of normalization; the use of
normalization form NFC is RECOMMENDED. Actually, if one is going to
do internationalization properly, one of the most often-cited goals
is to permit people to spell their names correctly. Since many
mailbox local parts reflect personal names, that principle applies as
well. And NFKC is not recommended because it may lose information
that is needed to correctly spell some names except in unusual
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circumstances.
4.2. Changes on MIME Headers
This specification updates Section 6.4 of [RFC2045]. [RFC2045]
prohibits applying a content-transfer-encoding to all subtypes of
message/. This specification relaxes the rule -- it allows newly
defined MIME types to permit content-transfer-encoding, and it allows
content-transfer-encoding for message/global (see Section 4.6).
Background: Normally, transfer of message/global will be done in
8-bit-clean channels, and body parts will have "identity" encodings,
that is, no decoding is necessary. In the case where a message
containing a message/global is downgraded from 8-bit to 7-bit as
described in [RFC1652], an encoding may be applied to the message; if
the message travels multiple times between a 7-bit environment and an
environment implementing UTF8SMTPbis, multiple levels of encoding may
occur. This is expected to be rarely seen in practice, and the
potential complexity of other ways of dealing with the issue are
thought to be larger than the complexity of allowing nested encodings
where necessary.
4.3. Syntax Extensions to RFC 5322
The following rules are intended to extend the corresponding rules in
[RFC5322] in order to allow UTF-8 characters.
FWS = <see [RFC5322], folding white space>
CFWS = <see [RFC5322], folding white space>
ctext =/ UTF8-xtra-char
utext =/ UTF8-xtra-char
comment = "(" *([FWS] utf8-ccontent) [FWS] ")"
word = utf8-atom / utf8-quoted-string
This means that all the [RFC5322] constructs that build upon these
will permit UTF-8 characters, including comments and quoted strings.
We do not change the syntax of <atext> in order to allow UTF-8
characters in <addr-spec>. This would also allow UTF-8 characters in
<message-id>, which is not allowed due to the limitation described in
Section 4.5. Instead, <utf8-atext> is added to meet this
requirement.
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utf8-text = %d1-9 / ; all UTF-8 characters except
%d11-12 / ; US-ASCII NUL, CR, and LF
%d14-127 /
UTF8-xtra-char
utf8-quoted-pair = ("\" utf8-text) / obs-qp
utf8-qcontent = utf8-qtext / utf8-quoted-pair
utf8-quoted-string = [CFWS]
DQUOTE *([FWS] utf8-qcontent) [FWS] DQUOTE
[CFWS]
utf8-ccontent = ctext / utf8-quoted-pair / comment
utf8-qtext = qtext / UTF8-xtra-char
utf8-atext = ALPHA / DIGIT /
"!" / "#" / ; Any character except
"$" / "%" / ; controls, SP, and specials.
"&" / "'" / ; Used for atoms.
"*" / "+" /
"-" / "/" /
"=" / "?" /
"^" / "_" /
"`" / "{" /
"|" / "}" /
"~" /
UTF8-xtra-char
utf8-atom = [CFWS] 1*utf8-atext [CFWS]
utf8-dot-atom = [CFWS] utf8-dot-atom-text [CFWS]
utf8-dot-atom-text = 1*utf8-atext *("." 1*utf8-atext)
qcontent = utf8-qcontent
To allow the use of UTF-8 in a Content-Description header field
[RFC2045], the following syntax is used:
description = "Content-Description:" unstructured CRLF
The <utext> syntax is extended above to allow UTF-8 in all
<unstructured> header fields.
Note, however, this does not remove any constraint on the character
set of protocol elements; for instance, all the allowed values for
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timezone in the Date: headers are still expressed in ASCII. And
also, none of this revised syntax changes what is allowed in a
<message-id>, which will still remain in pure ASCII.
4.4. Change on addr-spec Syntax
Internationalized email addresses are represented in UTF-8. Thus,
all header fields containing <mailbox>es are updated to permit UTF-8
addresses.
mailbox = name-addr / addr-spec / utf8-addr-spec
angle-addr =/ [CFWS] "<" utf8-addr-spec">" [CFWS] /
obs-angle-addr
utf8-addr-spec = utf8-local-part "@" utf8-domain
utf8-local-part= utf8-dot-atom / utf8-quoted-string / obs-local-part
utf8-domain = utf8-dot-atom / domain-literal / obs-domain
Below are a few examples of possible <mailbox> representations.
"DISPLAY_NAME" <ASCII@ASCII>
; traditional mailbox format
"DISPLAY_NAME" <non-ASCII@non-ASCII>
; message will bounce if UTF8SMTPbis extension is not supported
<non-ASCII@non-ASCII>
; without DISPLAY_NAME and quoted string
; message will bounce if UTF8SMTPbis extension is not supported
4.5. Trace Field Syntax
"For" fields containing internationalized addresses are allowed, by
use of the new uFor syntax. UTF-8 information may be needed in
Received fields. Such information is therefore allowed to preserve
the integrity of those fields. The uFor syntax retains the original
UTF-8 email address between email address internationalization EAI-
aware MTAs.
The "Return-Path" header field provides the email return address in
the mail delivery. Thus, the header is augmented to carry UTF-8
addresses (see the revised syntax of <angle-addr> in Section 4.4 of
this document). This will not break the rule of trace field
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integrity, because the header field is added at the last MTA and
described in [RFC5321].
The <item-value> on "Received:" syntax is augmented to allow UTF-8
email address in the "For" field. <angle-addr> is augmented to
include UTF-8 email address. In order to allow UTF-8 email addresses
in an <addr-spec>, <utf8-addr-spec> is added to <item-value>.
item-value =/ utf8-addr-spec
4.6. message/global
Internationalized messages must only be transmitted as authorized by
[I-D.yao-eai-rfc5336bis] or within a non-SMTP environment which
supports these messages. A message is a "message/global message", if
o it contains UTF-8 header values as specified in this document, or
o it contains UTF-8 values in the headers fields of body parts.
The type message/global is similar to message/rfc822, except that it
contains a message that can contain UTF-8 characters in the headers
of the message or body parts. If this type is sent to a 7-bit-only
system, it has to be encoded in MIME [RFC2045]. (Note that a system
compliant with MIME that doesn't recognize message/global would treat
it as "application/octet-stream" as described in Section 5.2.4 of
[RFC2046].)
Type name: message
Subtype name: global
Required parameters: none
Optional parameters: none
Encoding considerations: Any content-transfer-encoding is permitted.
The 8-bit or binary content-transfer-encodings are recommended
where permitted.
Security considerations: See Section 5.
Interoperability considerations: The media type provides
functionality similar to the message/rfc822 content type for email
messages with international email headers. When there is a need
to embed or return such content in another message, there is
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generally an option to use this media type and leave the content
unchanged or down-convert the content to message/rfc822. Both of
these choices will interoperate with the installed base, but with
different properties. Systems unaware of international headers
will typically treat a message/global body part as an unknown
attachment, while they will understand the structure of a message/
rfc822. However, systems that understand message/global will
provide functionality superior to the result of a down-conversion
to message/rfc822. The most interoperable choice depends on the
deployed software.
Published specification: RFC XXXX
Applications that use this media type: SMTP servers and email
clients that support multipart/report generation or parsing.
Email clients which forward messages with international headers as
attachments.
Additional information:
Magic number(s): none
File extension(s): The extension ".u8msg" is suggested.
Macintosh file type code(s): A uniform type identifier (UTI) of
"public.utf8-email-message" is suggested. This conforms to
"public.message" and "public.composite-content", but does not
necessarily conform to "public.utf8-plain-text".
Person & email address to contact for further information: See the
Author's Address section of this document.
Intended usage: COMMON
Restrictions on usage: This is a structured media type which embeds
other MIME media types. The 8-bit or binary content-transfer-
encoding MUST be used unless this media type is sent over a 7-bit-
only transport.
Author: See the Author's Address section of this document.
Change controller: IETF Standards Process
5. Security Considerations
If a user has a non-ASCII mailbox address and an ASCII mailbox
address, a digital certificate that identifies that user may have
both addresses in the identity. Having multiple email addresses as
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identities in a single certificate is already supported in PKIX
(Public Key Infrastructure for X.509 Certificates) and OpenPGP.
Because UTF-8 often requires several octets to encode a single
character, internationalized local parts may cause mail addresses to
become longer. As specified in [RFC5322], each line of characters
MUST be no more 998 octets, excluding the CRLF.
Because internationalized local parts may cause email addresses to be
longer, processes that parse, store, or handle email addresses or
local parts must take extra care not to overflow buffers, truncate
addresses, or exceed storage allotments. Also, they must take care,
when comparing, to use the entire lengths of the addresses.
In this specification, a user could provide an ASCII alternative
address for a non-ASCII address. However, it is possible these two
addresses go to different mailboxes, or even different people. This
configuration may be based on a user's personal choice or on
administration policy. We recognize that if ASCII and non-ASCII
email is delivered to two different destinations, based on MTA
capability, this may violate the principle of least astonishment, but
this is not a "protocol problem".
The security impact of UTF-8 headers on email signature systems such
as Domain Keys Identified Mail (DKIM), S/MIME, and OpenPGP is
discussed in [I-D.ietf-eai-frmwrk-4952bis], Section 14.
6. IANA Considerations
IANA has registered the message/global MIME type using the
registration form contained in Section 4.4.
7. Acknowledgements
This document incorporates many ideas first described in Internet-
Draft form by Paul Hoffman, although many details have changed from
that earlier work.
The author especially thanks Jeff Yeh for his efforts and
contributions on editing previous versions.
Most of the content of this document is provided by John C Klensin.
Also, some significant comments and suggestions were received from
Charles H. Lindsey, Kari Hurtta, Pete Resnick, Alexey Melnikov, Chris
Newman, Yangwoo Ko, Yoshiro Yoneya, and other members of the JET team
(Joint Engineering Team) and were incorporated into the document.
The editor sincerely thanks them for their contributions.
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8. Edit history
This section is used for tracking the update of this document. Will
be removed after finalize.
8.1. draft-ietf-eai-rfc5335bis-00
1. Applied Errata suggested by Alfred Hoenes.
2. Adjust [RFC2821] and [RFC2822] to [RFC5321] and [RFC5322].
3. Abrogate <alt-address> in ABNF of <angle-addr>.
4. Revoke [RFC5504] from this document.
5. Upgrade some references from I-Ds to RFC.
9. References
9.1. Normative References
[I-D.ietf-eai-frmwrk-4952bis] Klensin, J. and Y. Ko, "Overview and
Framework for Internationalized
Email",
draft-ietf-eai-frmwrk-4952bis-00 (work
in progress), June 2010.
[I-D.yao-eai-rfc5336bis] Yao, J. and W. MAO, "SMTP Extension
for Internationalized Email Address",
draft-yao-eai-rfc5336bis-00 (work in
progress), July 2009.
[RFC1652] Klensin, J., Freed, N., Rose, M.,
Stefferud, E., and D. Crocker, "SMTP
Service Extension for 8bit-
MIMEtransport", RFC 1652, July 1994.
[RFC2119] Bradner, S., "Key words for use in
RFCs to Indicate Requirement Levels",
BCP 14, RFC 2119, March 1997.
[RFC2821] Klensin, J., "Simple Mail Transfer
Protocol", RFC 2821, April 2001.
[RFC2822] Resnick, P., "Internet Message
Format", RFC 2822, April 2001.
[RFC3629] Yergeau, F., "UTF-8, a transformation
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format of ISO 10646", STD 63,
RFC 3629, November 2003.
[RFC5198] Klensin, J. and M. Padlipsky, "Unicode
Format for Network Interchange",
RFC 5198, March 2008.
[RFC5234] Crocker, D. and P. Overell, "Augmented
BNF for Syntax Specifications: ABNF",
STD 68, RFC 5234, January 2008.
[RFC5321] Klensin, J., "Simple Mail Transfer
Protocol", RFC 5321, October 2008.
[RFC5322] Resnick, P., Ed., "Internet Message
Format", RFC 5322, October 2008.
9.2. Informative References
[RFC2045] Freed, N. and N. Borenstein,
"Multipurpose Internet Mail Extensions
(MIME) Part One: Format of Internet
Message Bodies", RFC 2045,
November 1996.
[RFC2046] Freed, N. and N. Borenstein,
"Multipurpose Internet Mail Extensions
(MIME) Part Two: Media Types",
RFC 2046, November 1996.
[RFC2047] Moore, K., "MIME (Multipurpose
Internet Mail Extensions) Part Three:
Message Header Extensions for Non-
ASCII Text", RFC 2047, November 1996.
[RFC5504] Fujiwara, K. and Y. Yoneya,
"Downgrading Mechanism for Email
Address Internationalization",
RFC 5504, March 2009.
[RFC5721] Gellens, R. and C. Newman, "POP3
Support for UTF-8", RFC 5721,
February 2010.
[RFC5738] Resnick, P. and C. Newman, "IMAP
Support for UTF-8", RFC 5738,
March 2010.
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Authors' Addresses
Abel Yang
TWNIC
4F-2, No. 9, Sec 2, Roosvelt Rd.
Taipei, 100
Taiwan
Phone: +886 2 23411313 ext 505
EMail: abelyang@twnic.net.tw
Shawn Steele
Microsoft
EMail: Shawn.Steele@microsoft.com
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